Loading platform systems of this type have long been known, and are used particularly on trucks, for unloading and loading at a loading station; in particular:
to lower freight items from the truck bed to the roadway on which the truck is parked, from whence the freight items are transported to their destination by other means; and also PA1 to load freight items onto a truck, wherewith the item is deposited on the lift-type loading platform when the platform is disposed at the roadway level, following which the platform is raised to the level of the truck bed, and the freight item is then transferred from the platform to the truck bed. When the process (loading or unloading) is completed, the loading platform is swung from the horizontal orientation to a vertical orientation, where it can serve to close off the truck bed and the generally box-shaped cargo compartment of the truck.
A so-called "lifting cylinder mechanism" is used to raise and lower the loading platform while maintaining the platform in an essentially horizontal orientation, for loading and unloading to and from the truck bed. Such a "lifting" cylinder mechanism may be powered by hydraulic cylinder or other means. A "closing cylinder mechanism", powered by hydraulic cylinder or other means, is employed to swing the loading platform back and forth between a horizontal orientation and a vertical orientation. Known types of systems for loading platforms, as described supra, thus have two cylinder mechanisms, one of which, the "lifting" cylinder mechanism, typically interacts with one of the supporting frames which are components of the combined, parallelogram-configured "lifting and closing" mechanism, and the other, the "closing" cylinder mechanism, typically interacts with the other supporting frame which is a component of said "lifting and closing" mechanism.
In known loading platform systems of the type described supra, in loading and unloading, when forces are exerted which may be substantial in the case of heavy loads, problems can arise if the load carried on the loading platform causes the loading platform to bend (or deflect) with respect to the (flat) load bed of the truck, resulting in both horizontal and vertical gaps between the loading platform and the truck bed; and the magnitude of such gaps may vary substantially in relation to the width of the cargo compartment opening. The result is that, in particular for a heavy load which must be transported vertically with the loading platform oriented in a horizontal position, i.e. during raising and lowering, the distortions and misalignments which occur can be tantamount to barriers between the plane of the loading platform and the load bed of the truck. To overcome the barrier, the load must be lifted by supplementary means (manually, or with a dolly, hand truck, or other means).
This expedient is hazardous to the goods being loaded, which may be damaged when they are tilted to facilitate the crossing of the barrier between the mounting loading platform and the truck bed; moreover, the personnel performing these maneuvers are at substantial risk of injury, e.g. serious compression injuries of extremities (feet and hands).
The horizontal and vertical gaps, which on one side may even be greater than the depth of the loading platform itself (in the longitudinal direction of the vehicle), arise from the fact that the "lifting" cylinder mechanism and the "closing" cylinder mechanism are both disposed at lateral distances from the "supporting frames" of which the "lifting and closing mechanism" is comprised, i.e. they are oriented perpendicularly to the opening of the truck cargo compartment of the truck bed and perpendicularly to the loading platform. This arrangement allows substantially leveraged forces to develop, under load conditions; and these forces result in the development of the aforementioned horizontal and vertical gaps under the load conditions.
An approach to a solution of this serious problem for conditions of moderate stress on the loading platform during raising and lowering of freight items has been to provide a structure in which the first and second supporting frames, which form the "lifting and closing mechanism", are spaced apart and are inclined at an angle with respect to each other, such that when the expected moderate freight load is present and being raised or lowered by the loading platform system, the "lifting and closing mechanism" yields flexurally in combination with the loading platform in a manner which compensates for the tendency to deform and misalign, with the result being that ultimately the loading platform is aligned flush with the plane of the truck bed. However, such a result is achieved only when the freight load borne is precisely that expected load for which the described ad hoc solution was designed. Moreover, the known solution has the disadvantage that when the loading platform no longer bears a freight load but is swung up into its vertical "closing" position in which it closes off the truck cargo compartment, it is not properly aligned to perform that closure, but rather a gap is present between the vertically oriented loading platform and the loading opening of the cargo compartment of the truck, which gap widens on both sides, due to a rearward angular sweep of the no longer planar loading platform. Another important disadvantage of the known solution is that if the loading platform is loaded with freight wherewith the applied stress from the load exceeds a specified stress value and the applied stress is less than the design stress, undesirable gaps between the truck bed and the plane of the loading platform again develop, with the above-described undesirable consequences.